1. Field of the Invention
The present invention pertains to a customizable seal that contacts a portion of a patient to provide a comfortable and customizable interface between an external device, such as a respiratory mask, and the patient. The present invention also pertains to a respiratory mask having such a customizable seal and to a method of interfacing a patient with an external device, such as a respiratory mask, using such a seal.
2. Description of the Related Art
A variety of respiratory masks are known having a flexible seal that covers the areas surrounding the nose and/or mouth of a human user and that are designed to create a continuous seal against the user's face. Because of the sealing effect created, gases can be provided at a positive pressure within the mask for consumption by the user. The uses for such masks range from high altitude breathing (aviation applications), swimming, mining and fire fighting applications and various medical diagnostic and therapeutic applications.
One requisite of many of these masks, particularly medical respiratory masks is that they provide an effective seal against the user's face to prevent leakage of the gas being supplied. Commonly, in conventional mask configurations, a good mask-to-face seal has been attained in many instances only with considerable discomfort for the user. This problem is most crucial in those applications, especially medical applications, which require the user to wear the mask continuously for hours or perhaps even days. In such situations, the user will not tolerate the mask for long durations and optimum therapeutic or diagnostic objectives will not be achieved, or will be achieved with great difficulty and considerable user discomfort.
Several types of respiratory masks for the types of applications mentioned above are known. Perhaps the most common type of mask incorporates a smooth sealing surface extending around the periphery of the mask and exhibiting a generally uniform, i.e., predetermined or fixed, seal surface contour that is intended to be effective to seal against the user's face when force is applied to the mask with the sealing surface in confronting engagement with the user's face. The sealing surface typically consists of an air or fluid filled cushion, or it may simply be a molded or formed surface of a resilient seal element made of an elastomer such as plastic, rubber, silicone, vinyl or foam.
Such masks have performed well when the fit is good between the contours of the seal surface and the corresponding contours of the user's face. This may occur, for example, if the contours of the user's face happen to match well with the predetermined contours of the seal. However, if the seal fit is not good, there will be gaps in the seal-to-face interface resulting in gas leaking from the mask at the gaps. Excessive force will be required to compress the seal member to close the gaps and attain a satisfactory seal in those areas where the gaps occur. Such excessive force is unacceptable because it produces high pressure points elsewhere on the face of the user where the mask seal contour is forcibly deformed against the face to conform to the user's facial contours. This will produce considerable user discomfort and possible skin irritation and breakdown anywhere the applied force exceeds the local perfusion pressure, which is the pressure that is sufficient to cut off surface blood flow. Ideally, contact forces should be limited between the mask and the user's face to avoid exceeding the local perfusion pressure, even at points where the mask seal must deform considerably.
The problem of seal contact force exceeding desirable limits is even more pronounced when the positive pressure of the gas being supplied is relatively high or is cyclical to relatively high levels. Because the mask seals by virtue of confronting contact between the mask seal and the user's face, the mask must be held against the face with a force sufficient to seal against leakage of the peak pressure of the supplied gas. Thus, for conventional masks, when the supply pressure is high, headstraps or other mask restraints must be relatively tightly fastened. This produces high localized pressure on the face, not only in the zone of the mask seal, but at various locations along the extent of the retention straps as well. This, too, will result in discomfort for the user after only a brief time. Even in the absence of excessive localized pressure points, the tight mask and headstraps may become extremely uncomfortable, and user discomfort may well cause discontinued cooperation with the treatment regimen. Examples of respiratory masks possessing continuous cushion sealing characteristics of the type just described are provided in U.S. Pat. Nos. 2,254,854 and 2,931,356.
U.S. Pat. No. 5,181,506 describes a protective gas mask for military applications. The mask includes a three-layer face piece, the central layer of which is a thick layer of relatively stiff material having preformed V-shaped channels. The channels are “overfilled” with a gel or both gel and compressed air to create bulges in an inner face-contacting layer that are adapted to seal against the contours of a user's face. The inherent stiffness of the central layer in combination with the structural rigidity provided by the V-shaped channels, especially when overfilled with gel/air, results in a comparatively unyielding facial seal. Indeed, the mask is deployed in combination with a tightly fitting hood in order to draw the face piece firmly against the user's head to generate the desired facial seal. As will be appreciated, the comfort afforded such a construction is quite limited and certainly not appropriate for those applications, such as respiratory therapy situations, where a user must occasionally wear a mask for prolonged periods of time.
Several classes of cushion materials, including gels and foams, were analyzed in a study by S. F. C. Stewart, V. Palmieri and G. V. B. Cochran, Arch. Phys. Med. Rehabil., Vol. 61, (May 1980). That study compared the relative advantages and disadvantages of such cushion materials when used as wheelchair cushions, specifically the effects of such materials on skin temperature, heat flux and relative humidity at the skin-cushion interface. Each of these factors, along with applied pressure in excess of local perfusion pressure, has been identified as a contributor to breakdown of skin tissue at the skin-cushion interface.
In that study, foam cushions were reported to increase skin temperatures by several degrees after a few hours of use. This was suggested to be a result of the comparatively low heat flux characteristics of foam materials. That is, the foam materials and the air entrapped within them tend to be poor conductors of heat. Conversely, gel pads, as a group, showed a considerably higher heat flux than foam, sufficient, in fact, to maintain skin temperatures relatively constant after several hours of use. The sole benefit of foam versus gel reported in the study was that foams produced lesser relative humidity than gels at the skin-cushion interface. This was attributed to the open cell structure of the foams which provide a pathway through which moisture can diffuse. This seeming advantage is somewhat problematic, however, in that open cell foam tends to promote bacteria growth when exposed to perspiration. Bacteria, in turn, contaminates the foam thereby considerably hindering its useful service life.
These and other detrimental characteristics have been observed as well in the foam-type respiratory mask seals discussed above. Hence, apart from generally failing to provide optimum sealing with respect to a user's face, the inherent qualities of foam mask seals have been linked to skin irritation and breakdown, particularly at some of the more prominent facial contours, such as the cheek bones and bridge of the nose.
Moreover, whether air, fluid or, in the case of U.S. Pat. No. 5,181,506, gel filled, or whether formed as an elastomer such as foam, plastic, rubber, silicone and the like, the resiliency or recoil characteristics of presently available cushion type respiratory mask seals have not been well suited to form an effective seal with the topography of the user's face in the absence of considerable headstrap tensile forces.
One method to reduce the existence of gaps at the mask-to-face interface is to customize the seal so that it conforms to the fine contours of the patient's face. This can be thought of as a micro-customization of the seal because the goal of the customization is to match the seal to the specific external features of the user's face, i.e., the contours created by the soft tissue of the patient. For example, if the user has an unusually deep crease in his or her face, a micro-customized mask has a user interface surface that matches this deep crease, thereby preventing a gap from existing at the crease. In short, a micro-customized seal is tailored to conform to the contours of the soft surface tissue of the patient.
Various techniques have been proposed for micro-customizing a seal, such as the seal on a face mask. It is known, for example, to provide a micro-customized seal by making an impression or cast of the patient's face. The cast is then used as a form to produce a fully customized mask specifically tailored to match that patient's face. This technique, however, is time consuming and costly, and, therefore, is not well suited for conventional, large-scale manufacturing processes.
The present inventor also discovered that, contrary to expected results, a satisfactory seal may not result from a micro-customized mask. It is believed that a relatively detailed micro-customized mask, closely matching the detailed contours of the soft tissue at the surface of a patient, does not provide a satisfactory seal because changes in seal position and/or changes in the soft tissue of the patient may result in new gaps being created between the seal and the patient. For this reason, a mask that has a micro-customized seal made from the above-described casting process, because it is specifically designed to match the contours in the soft tissue of the patient's face at the time the cast was made, typically does not have the ability, or has only a limited ability, to change its shape in the event of changes in the patient's shape or shifts in the mask position. This disadvantage is especially pronounced if the mask having such a micro-customized seal is used in situations where the patient is likely to move and/or in situations where the mask is likely to be jostled, such as during sleep.
It is also known to contour the patient-contacting surface of the mask to match the general facial contours of the patient. This can be thought of as a macro-customization because the goal of customization is not to match the seal to the detailed external features of the user defined by the external soft tissues, but to match the seal to the general shape of the user, such as the underlying bone structure. Macro-customization provides an advantage over micro-customization in that there is less of a chance that changes in the patient's soft tissue or slight shifts in the seal will result in gaps being created. Also, a macro-customized seal provides a more effective seal than a micro-customized seal in situations where there may be differences between the contours of the underlying bone structure and the overlying soft tissue. For example, if there is a protruding bone that is not apparent because the protrusion is masked by soft tissue, a macro-customized seal will conform to the protruding bone structure, thereby minimizing the chances of leaks existing at a site near the protruding bone.
One technique for providing a macro-customized seal on a respiratory mask is to provide a variety of different masks having a variety of differently shaped seals. The user would use the mask having the seal that most closely matches the facial structure of that user. For example, several masks having different sized nose bridge arches can be made available to the user, with the user selecting the mask having the nose bridge arch size that most closely matches his or her nose. This type of mask provides some degree of customization, as opposed, for example, to a flat surface, for the mask-to-patient interface. However, because this macro-customized, i.e., off the shelf, mask is not specifically customized to match the facial features of a specific user, it often does not permit a sufficient degree of customization to account for facial contours specific to each patient. For example, for patients with unusual facial features, off the shelf macro-customized masks typically do not provide a satisfactory seal and can result in pressure points being created as the patient attempts to close these gaps with increased strapping force.
Macro-customization of a respiratory mask facial seal can also be accomplished by measuring the general facial features of the patient and producing a seal that matches these general features. This macro-customization process, however, suffers from the same disadvantages discussed above with respect to the micro-customization process. Namely, it is time consuming, uneconomical and inefficient to attempt to mass produce such specifically tailored macro-customized masks.
Macro-customization also suffers from a disadvantage in that leaks resulting from the physical characteristics of the soft tissue of the patient are not minimized. For example, if there is a deep crease in the soft tissue, a macro-customized seal is generally not as prone to reducing leaks at the crease as a micro-customized seal. Instead, the user will typically attempt to minimize such leaks by increasing the strapping force, thereby creating the problems of high localized pressure on the surface of the patient.